In processing machines, in particular printing presses, a material web is moved along driven axles (web transport axles), such as pull rolls or advancing rolls, and non-driven axles, such as deflection, guiding, drying or cooling rolls. At the same time, the material web is processed, for example printed, punched, cut, folded, etc., by means of usually likewise drive processing axles.
The web tension of the material web is influenced, for example, via what are known as clamping points which clamp the material web positively or non-positively. There are as a rule driven transport or processing units. In a gravure printing press, a clamping point is usually formed by a printing unit, in which a frictional unit exists between the driven impression cylinder, the impression roller and the material web. The material web is divided into web tension sections, a web tension section being delimited by two clamping points. Further driven and/or non-driven axles can be arranged within a web tension section. The entire material web is often divided into a plurality of web tension sections, sometimes also with different web tension setpoint values. A web tension regulating means is usually used to maintain the setpoint values.
The adjustment or regulation of the web tension of a web tension section can take place by way of different methods. Downstream means that the clamping point which delimits the downstream end of the web tension section is adjusted, and upstream means that the clamping point which delimits the upstream end of the web tension section is adjusted. In this simple embodiment, however, the web tension in leading and/or trailing web tension sections is not decoupled from the actuating movement. Rather, the change in the web tension is transported through the machine such that it follows the material web course, and is to be adjusted in all following sections. In addition to this indirect disruption on account of the transport of the material web, a direct disruption on account of the actuating movement is to be found in the web section which adjoins the adjusted clamping point.
It is possible, in the case of a downstream regulation by means of a (dynamic) downstream pilot control, to pilot control all the following clamping points in such a way that they directly compensate for the effects of the leading clamping point. As a consequence, this ensures that all the following web tension regulators do not have to adjust the disruptions of the actuating movement and of the coupling by the material web.
DE 10 2008 056 132 A1 proposes decoupling for an upstream regulation, a (dynamic) downstream pilot control by means of PT1 element also being carried out in addition to a (constant) upstream pilot control.
DE 10 2009 016 206 A1 discloses a method with decoupling which is implemented exclusively in the upstream direction. Here, a combination of pilot control weighted in the upstream direction by a DT1 element and pilot control weighted by a negative PT1 element exclusively of the rear delimiting clamping point is disclosed. The specifications “upstream of” and “downstream of” a clamping point or material web section relate to the transport direction of the material web, that is to say the material web course.
All known decoupling strategies have the disadvantage that the web running speed changes within the web sections. Since, however, for example in digital printing, the print heads are sited within a web section and not directly at a clamping point, every actuating movement during the regulation of the web tensile force leads as a rule to a change in the material web speed and therefore to a register error.
It is therefore desirable to specify a method for web tension setting in a processing machine, in which method the material web speed can be kept constant or can be changed in a targeted manner during the decoupling at any desired point within a web tension section.